WKB treatment for unstable particle I was wondering if the WKB treatment of particles entering a potential (there is some reflection (R) and transmission (T) coefficient such that R+T =1) works only for stable particles. Essentially the WKB gives me a wave function of a state and the probability it will be reflected/transmitted across a potential. 
What is I have an unstable intermediate particle, do I always need to use the stable final states?
Thanks!
 A: This is not a complete answer, but is too long for a comment.
Interesting question. I guess spontaneous fission would be an example, since it takes place by tunneling through a barrier. (The coordinate is some measure of the shape of the nucleus, leading to a saddle point and then scission.) People certainly do use WKB concepts to discuss fission, and the products are often unstable nuclei. So I think the answer is that it's at least sometimes OK.
My gut feeling would be that you'd run into problems if the half-life of the product was too short. The fission example would seem to indicate that it can be OK even if the half-life of the product is short compared to the rate of transmission. Perhaps there would be a problem if the half-life of the product was short compared to one over the frequency of assaults on the barrier (although such a thing is not actually very well-defined in examples like fission and alpha decay, since preformation of the product would violate the exclusion principle).
